Bromide removal

ABSTRACT

A chlorination system in which brine is converted to sodium hypochlorite by an electrolyser. Brine ( 1 ) fed to the electrolyser ( 3 ) is passed through a filter ( 2 ) which is capable of adsorbing bromine or hypobromous acid. Some of the sodium hypochlorite produced in an electrolyser ( 3 ) is fed back to a point in the brine feed upstream of the filter ( 2 ) such that any bromide in the brine is oxidized to bromine or hypobromous acid and therefore adsorbed by the filter ( 2 ).

BACKGROUND OF THE INVENTION

The present invention relates to a method and apparatus for removingbromide from an electrolytic chlorination system.

BACKGROUND OF RELATED ART

It is well known to produce sodium hypochlorite from sodium chloridebrine by converting the brine to sodium hypochlorite in an electrolyser.Sodium hypochlorite is used to treat potable water. Unfortunately, ifthe brine includes traces of bromide, the electrolyser causes theconversion of the bromide by oxidation to bromate. It is desirable toproduce bromate-free sodium hypochlorite for the treatment of potablewater as experiments have indicated that bromate may be carcinogenic.

It is an object of the present invention to provide a method andapparatus for removing bromide from an electrolytic chlorination systemso as to obviate or mitigate the problem outlined above.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a method forremoving bromide from an electrolytic chlorination system in which brineis converted to sodium hypochlorite in an electrolyser, wherein thebrine is fed to the electrolyser through a filter containing a mediumcapable of adsorbing bromine or hypobromous acid, and sodiumhypochlorite is fed back from the electrolyser and mixed with the brinefeed upstream of the filter to oxidize any bromide in the brine tobromine or hypobromous acid.

The present invention also provides an electrolyser for converting brineto sodium hypochlorite, means for feeding brine to the electrolyser, afilter through which the brine is fed to the electrolyser, the filterbeing capable of adsorbing bromine or hypobromous acid, and means formixing sodium hypochlorite from the electrolyser with the brine upstreamof the filter such that bromide in the brine is oxidized to bromine orhypobromous acid. Preferably hydrochloric acid is mixed with the brineupstream of the filter to maintain a low pH and thereby ensure effectiveoxidation of the bromide.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing:

FIG. 1 is a schematic illustration of a system according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the present invention will now be described, by way ofexample, with reference to the accompanying drawing which illustrates abromide removal system incorporated into a conventional on-siteelectrolytic chlorination system.

The illustrated electrolytic chlorination system comprises a saltsaturator 1 in which brine is prepared. The saturator 1 may have acapacity of, for example, 15 cubic meters. Brine flows from thesaturator 1 through a filter 2 to an electrolyser 3, the brine flowbeing maintained by a brine transfer pump 4. A flow rate through thepump 4 of 51.5 liters per hour may be established. The electrolyser 3 isof conventional form and is effective to oxidize the brine to sodiumhypochlorite. The contents of the electrolyser are mixed with dilutionwater supplied through line 5 at a rate of 540 liters per hour, theresultant sodium hypochlorite being transferred to a product tank 6 witha capacity of 13 cubic meters.

With the exception of the filter 2, the components shown in the drawingand described above are entirely conventional. With the operation ofsuch a conventional system, however, traces of bromide in the brine floware converted to bromate and reach the product tank 6. This isundesirable as the content of the product tank 6 is delivered via line 7to a treatment plant (not shown) in which the contents of the product'stank is mixed with a supply of potable water.

In accordance with the present invention, sodium hypochlorite from theproduct tank 6 is fed back through line 8 to a point upstream of thefilter 2. The flow of hypochlorite is maintained by a hypochloritetransfer pump 9. Hydrochloric acid is also delivered to the brine flowupstream of the filter 2 via line 10, the flow of hydrochloric acidbeing maintained by a pump 11. In the illustrated system the flow ofsodium hypochlorite was 1 liter per hour and the flow of hydrochloricacid was 1 liter per hour. The supply of hydrochloric acid is controlledto maintain a low pH in the brine flow upstream of the filter 2.Typically the acidity will be controlled to approximately 4 pH. At sucha low pH, bromide in the brine flow is rapidly converted into bromineand hypobromous acid which is adsorbed by the filter 2. By appropriateselection of the stoichiometry and reaction time the further oxidizationof the hypobromous acid to bromate can be minimized or substantiallyeliminated. Accordingly, providing the filter is capable of adsorbingbromine or bromine released by the reduction of hypobromous acid,substantially no bromine compounds reach the electrolyser andaccordingly substantially no bromate reaches the product tank 8.

Appropriate materials for the filter 2 are activated carbon zeolites andinsoluble reducing agents, for example calcium sulphite.

What is claimed is:
 1. A method, comprising: providing a brine solutioncontaining bromide; oxidizing the bromide in the brine solutionfiltering the brine solution through an adsorption medium to produce asubstantially bromine-free solution; and passing the substantiallybromine-free solution through an electrolyzer to produce bromate-freehypochlorite.
 2. The method of claim 1, wherein the oxidation of thebromide produces bromine.
 3. The method of claim 1, wherein theoxidation of the bromide produces hydrobromous acid.
 4. The method ofclaim 1, wherein the bromide is oxidized using a source of hypochlorite.5. The method of claim 4, wherein the source of hypochlorite is thebromate-free hypochlorite.
 6. The method of claim 1, further comprisingreducing the pH of the brine solution before oxidizing the bromide. 7.The method of claim 6, wherein the pH of the brine solution is reducedby adding hydrochloric acid.
 8. The method of claim 1, wherein theadsorption medium is selected from the group consisting of activatedcarbon, zeolites and insoluble reducing agents.
 9. A system, comprising:an inlet for receiving a brine solution containing bromide; a source ofoxidizing agent fluidly connected to the inlet for oxidizing the bromideto produce bromine or hypobromous acid; a filter positioned downstreamof the source of oxidizing agent, adapted to adsorb bromine orhypobromous acid; an electrolyzer positioned downstream of the filter,for producing hypochlorite from the filtered brine solution; and anoutlet for discharging a bromate-free hypochlorite solution.
 10. Thesystem of claim 9, wherein the oxidizing agent includes hypochlorite.11. The system of claim 10, wherein the source of hypochlorite isconnected to the outlet.
 12. The system of claim 9, further comprising asource of acid fluidly connected to the system upstream of the filter.13. The system of claim 9, wherein the filter comprises a sorbing mediaselected from the group consisting of activated carbon, zeolites, andinsoluble reducing agents.